A continuously optically pumped (CW optically pumped) optical amplifier (resonator) for delivering pulsed optical radiation is typically held in an operation-ready state with optical pump-light delivered continuously to a gain-medium of the optical amplifier. When the amplifier is not actually operating, i.e., not delivering pulses, the gain-medium becomes saturated as pump-energy is being delivered to the gain-medium without any pulse energy being removed from the gain-medium.
Pulsed lasers that deliver pulses having a duration on the order of nanoseconds (ns) at frequencies of up to a few tens of kilohertz, usually include an optical resonator with an intra-resonator acousto-optical switch that can cause resonator losses to be variable in response to an applied RF potential. In the operation-ready state, the optical switch is in a “closed” mode such that the switch creates such optical losses in the resonator in a manner so that generation of pulsed radiation is not possible. Pulsed operation of the resonator is caused by periodically fully opening (no losses) and closing the optical switch. Absent any preventive measure, when the optical switch is opened for the first time in a sequence of openings and closings thereof to generate a train of pulses, all of the saturated gain is available and is extracted by the first pulse. The time between the delivery of the first pulse and the second pulse and between pulses thereafter is usually too short to allow the gain-medium to saturate, and these pulses will each have energy less than the first pulse.
In most applications of laser pulses, it is desirable that all pulses in a train thereof have about the same energy or peak power. This being the case, some measure must be taken to prevent the first pulse in a pulse-train from being more powerful than subsequent pulses in the pulse-train. In pulsed resonators using acousto-optical switches, one common measure to reduce the energy of the first pulse is to establish an operation-ready mode with the switch partially open to an extent that the resonator generates low-power CW laser radiation in response to the CW optical pumping. This prevents the gain-medium from saturation, and the extent of the switch opening can be adjusted such that the available gain is “clamped” at a about level that would be reached in a pulse-repetition period. Once the switch begins periodically fully-opening and closing at the pulse-repetition frequency, the first and subsequent pulses have about the same energy.
In apparatus for delivering trains of very short pulses, for example, pulses having a duration of less about a few picoseconds (ps), it is usual to employ a seed-pulse laser to generate low-power pulses (seed-pulses) and deliver these low power pulses to a regenerative amplifier for amplification. A regenerative amplifier usually has a resonator including a CW pumped gain-medium and a polarizing beam splitter cooperative with a half-wave rotator and a very fast electro-optical polarization-rotating switch such as a Pockels cell.
In one common arrangement, a seed pulse is transmitted into the resonator through the polarizing beamsplitter and the Pockels cell is then switched to rotate the polarization of the pulse so that the pulse can no longer be transmitted by the polarizing beamsplitter, thereby trapping the pulse in the resonator. The pulse circulates in the resonator and is amplified as it extracts energy from the gain-medium. After a predetermined number of round trips of the pulse in the resonator, the Pockels cell is switched such that the polarization-plane of the pulse rotates back to the orientation that is transmitted by the polarizing beamsplitter, and is delivered through the polarizing beamsplitter, out of the resonator. The switching operation (admitting seed-pulses and delivering amplified pulses) is repeated throughout the pulse sequence.
In a regenerative amplifier, the first pulse in a sequence will also usually have a much higher power than remaining pulses of the sequence. In the regenerative amplifier arrangement, however, the Pockels cell based optical switching arrangement can not be operated in the same manner as an acousto-optical switch to cause low level CW operation for gain-clamping. There is a need for a method of gain-clamping in a regenerative amplifier.